The present invention relates, in general, to kraft pulping chemical recovery processes, and, in particular, to a new and useful apparatus for recovering sulfur from concentrated acid gases.
In the kraft pulping process, wood chips are reacted with a strong solution of alkali and sulfur which is known as white liquor, to liberate papermaking fibers. The spent liquor from the pulping operation (black liquor) consists of the dissolved wood components and partially deactivated chemicals. In the conventional kraft chemical recovery process, the sodium and sulfur are recovered as sodium carbonate (Na2CO3) and sodium sulfide (Na2S) in the molten smelt generated by the combustion of black liquor in Tomlinson recovery boilers. The smelt is tapped from the boiler hearth and dissolved with a weakly alkaline process water (weak wash) to form green liquor. Sodium carbonate in green liquor is causticized by reaction with lime, CaO, to form sodium hydroxide (NaOH). The byproduct CaCO3 solids are separated and thermally decomposed in a lime kiln or similar device to recover CaO. The clarified solution of NaOH and Na2S is the white liquor which is returned to the pulping operation.
Gasification is an alternative to the Tomlinson boiler operation whereby black liquor is partially combusted with a sub-stoichiometric amount of oxygen to convert the organic portion of the liquor into a fuel gas which can be burned, for example, in a gas turbine. Such an arrangement is expected to increase the overall energy efficiency of the kraft pulping operation. One of the challenges to gasification is recovery of the sodium and sulfur from the black liquor in ways that do not adversely affect the balance of pulp mill operations.
In all black liquor gasification processes, some of the sulfur content of the black liquor ends up as hydrogen sulfide (H2S) in the fuel gas. Most of the sodium and the remaining sulfur leave the gasifier as molten smelt (or solid ash in the case of solid-phase gasification). The smelt contains mostly Na2CO3 and some Na2S; these chemicals are easily recovered by dissolution in weak wash or other process water streams. Various schemes have been proposed to recover H2S directly from the fuel gas by absorption in alkaline process streams. See, for example, U.S. Pat. No. 4,808,264 to Kignell; U.S. Pat. No. 5,284,550 to Tanca, et al.; U.S. Pat. No. 5,556,605 to Stigsson; and U.S. Pat. No. 5,634,950 to Mcllroy, et al.
There is an inherent difficulty in these approaches because the fuel gas contains a much greater amount of CO2 relative to H2S, and the carbon dioxide is subject to absorption by sodium hydroxide during H2S scrubbing operations. Co-absorption of CO2 thus increases lime demand in the causticizing operation which would, as a minimum, require additional fuel input to the lime kiln for calcining the CaCO3 byproduct and could potentially require additional kiln capacity if lime processing is limited.
The present invention is a critical component for minimizing adverse impacts on causticizing operations with black liquor gasification processes.
An object of the present invention is to provide a gas-liquid contacting device in which an acidic sulfur gas (H2S or SO2) is absorbed selectively from a gas stream containing the sulfur gas and CO2 into an aqueous alkaline solution. The invention minimizes the co-absorption of CO2 into the alkaline solution. The apparatus of the invention is intended to function as a component of pulp and paper mill chemical recovery processes based on spent liquor gasification. The spent liquor consists of the dissolved wood components and partially deactivated chemicals.
The gasification process operates at elevated pressure and utilizes a physical solvent scrubber/stripper system to separate a stream of concentrated acid gas (consisting substantially of H2S and CO2) from the fuel gas components (H2, CO, CH4) generated by the partial combustion of spent liquor with a sub-stoichiometric amount of oxidant. The concentrated acid gas is passed to the present invention where it contacts the sorbent; an aqueous solution of sodium hydroxide with lesser amounts of dissolved sodium sulfide, sulfate, carbonate, bicarbonate and chloride. This alkaline liquor is provided from a causticizing operation in which a stream of dissolved sodium carbonate is reacted with quicklime (CaO) in the conventional manner.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. This application is related to, but distinct and separate from, the processes disclosed in U.S. application Ser. Nos. 09/298,533 and 09/298,974, in which both Mr. Downs and Mr. Verrill are also listed as co-inventors for each application. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.